A fiber-reinforced plastic structure having a closed cross-section is widely used in a range from large structures such as the body or wings of an aircraft to small structures such as a bicycle frame, tennis racket, a fishing rod, and a golf shaft. In addition, a fiber-reinforced plastic structure having an open cross-section is widely used for helmets and the like.
As a core for forming the closed cross-section, a core formed into a predetermined shape by wrapping a powder particles and/or a group of grains (herein after, a powder grain group) in a packaging film and vacuum-packaging the result, a core using a molded product formed by blow molding, and the like are used. In a related art in which the core having the vacuum-packaged powder grain group formed into a predetermined shape is used, for example, a molded body having a hollow portion and a molding method for the same which are disclosed in JP 2-238912 A (Patent Document 1) are proposed, and as the core formed by the blow molding, for example, a multilayer plastic molded body and a manufacturing method for the same of JP 7-100856 A (Patent Document 2) are proposed.
As Conventional Example 1 of the invention, the invention described in Patent Document 1 will be described with reference to FIGS. 8 and 9. FIG. 8 illustrates an intermediate state in molding a structure having a hollow portion which is a type of closed cross-section by a mold 30. That is, FIG. 8 illustrates a state where a sheet-like fiber-reinforced thermoplastic resin material (lower FRTP) 34 which is subjected to pre-heating to be in a molten state is placed on a lower mold 31 of the molding mold 30. Since the lower FRTP 34 is in the molten state, the lower FRTP 34 is hung under its own weight and enters a state of being depressed in the concave portion of the lower mold 31. FIG. 9 schematically illustrates the state illustrated in FIG. 8.
A core 33 formed by wrapping a powder grain group 33a in a packaging material 33b and solidifying the result into a predetermined shape by vacuum packaging basically has a structure illustrated in FIG. 9 and is placed in the concave portion of the lower FRTP 34 that is hung under its own weight in the molten state. On the upper portion of the lower FRTP 34 in which the core 33 is placed, another sheet-like FRTP, the upper FRTP 35 which is pre-heated to be in a molten state is placed. In this state, the periphery of the core 33 is in a state of being enclosed by the lower FRTP 34 and the upper FRTP 35.
An upper mold 32 of the mold 30 is lowered from this state to pressurize the upper FRTP 35 and the lower FRTP 34 between the upper mold 32 and the lower mold 31 to integrally solidify such that the upper FRTP 35 and the lower FRTP 34 are integrally molded in a state of wrapping the core 33 therein. In order to discharge the core 33 from the semi-finished product completed here, small holes are bored through the semi-finished product. When holes are bored through the semi-finished product, air infiltrates into the powder grain group 33a of the vacuum-packaged core 33 and the binding of the powder grain group 33a is loosened.
In addition, the powder grain group 33a included in the core 33 is discharged to the outside of the semi-finished product through the small holes formed in the semi-finished product, thereby completing a molded article. If the packaging material 33b which vacuum-packages the powder grain group 33a is made of a material having good releasability from the molded article, the packaging material 33b may be easily detached from the molded article.
As Conventional Example 2 of the invention, the invention described in Patent Document 2 will be described with reference to FIG. 10. FIG. 10 illustrates a state where a core molded by blow molding is set between molds 41a and 41b for molding an outer layer. As illustrated in FIG. 10, the molds 41a and 41b are configured to accommodate a core 43, and when the molds 41a and 41b are closed, a cavity as a hollow portion is formed to be filled with a molten resin between respective molding surfaces 42a and 42b of the molds 41a and 41b and the core 43.
Molten resin 45 which is plasticized by an extruder 44 is supplied into the cavity. By supplying the molten resin 45 into the cavity of the molds 41a and 41b in the closed state, a product having a hollow portion can be molded into a desired shape. However, in a case where the heat resistance of the core 43 is low compared to the temperature of the molten resin or in a case where the thickness of the core 43 is small, the core 43 may be deformed depending on the pressure applied to the core 43 at the time of molding. In addition, in a case a wide flat part is present as the shape of the core 43, the rigidity of the flat part is likely to be insufficient, and thus the core 43 may be deformed.
In order to prevent the deformation of the core 43, in the invention described in Patent Document 2, a configuration in which the internal pressure of the core 43 can be increased is employed. As a configuration for this, a pressurizing unit 46 which communicates with the inside of the core 43 is provided, and pressurized gas or liquid is introduced into the core 43 from the pressurizing unit 46 to increase the internal pressure of the core 43.